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Evaluation of a Prototype Plastic Scintillator Detector for Patient-Specific QA of Volumetric Modulated Arc Stereotactic Radiosurgery Plans

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J Snyder

J Snyder*, R Popple , X Wu , Univ Alabama Birmingham, Birmingham, AL


TH-AB-FS1-7 (Thursday, August 3, 2017) 7:30 AM - 9:30 AM Room: Four Seasons 1

Purpose: To evaluate a prototype scintillator detector for applications to patient-specific QA. The prototype W2 (Standard Imaging, Middleton, WI) is a 1 mm³ scintillator coupled to an optical fiber.

Methods: Measurements were done using a water-equivalent phantom. The gantry was varied to measure the calibration of the W2 system as a function of incident beam angle. A series of square fields having known dose were delivered to the scintillator. A subset of these tests was repeated to evaluate the detectors response to radiation induced changes to scintillator and optical fiber. Four clinical SRS plans for single targets ranging in size from 3.12 to 0.11 cm³ were measured with Gafchromic EBT-XD (Ashland, Bridgewater, NJ) film, W2, and A16 ion chamber (Standard Imaging, Middleton, WI).

Results: The calibration for the W2 system was invariant when the gantry was rotated around the axis of the fiber, but when rotated along the axis of the fiber the calibration parameters varied by 4%. This difference is primarily due to changes in Cerenkov light generated in the fiber. Throughout the testing of the W2 the calibration parameterization has shifted by up to 10% due to radiation induced changes to the scintillator and optical fiber. The ratio of A16 to film was 1.00, 0.98, 0.98 and 0.97, for W2 the ratio was 1.01, 1.00, 0.99, and 0.97 for targets of size 3.12, 1.57, 0.48, and 0.11 cm³, respectively.

Conclusion: For patient-specific QA measurements individual calibrations are necessary for coplanar and vertex arcs. Changes in response suggest daily calibration is necessary. Additional work is ongoing to characterize the detector's response for small field VMAT radiosurgery plans.

Funding Support, Disclosures, and Conflict of Interest: Prototype W2 was loaned by Standard Imaging for evaluation.

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